feat: add collection count range assertions

[thomhurst]

Summary

• Adds HasAtLeast(int minCount) assertion: asserts collection has >= minCount items
• Adds HasAtMost(int maxCount) assertion: asserts collection has <= maxCount items
• Adds HasCountBetween(int min, int max) assertion: asserts collection count is between min and max (inclusive)
• Implemented for both IEnumerable<T> and IAsyncEnumerable<T> following existing patterns
• All assertions support chaining via .And / .Or continuations

Closes #4869

Test plan

• Verify HasAtLeast passes when collection count >= minCount and fails when count < minCount
• Verify HasAtMost passes when collection count <= maxCount and fails when count > maxCount
• Verify HasCountBetween passes when min <= count <= max and fails otherwise
• Verify chaining works (e.g., .HasAtLeast(2).And.HasAtMost(5))
• Verify async enumerable variants work correctly
• Verify null collection handling produces correct failure messages
• Run public API snapshot tests and update verified files if needed

[claude]

Code Review

Issue: Missing Public API Snapshot Updates

The PR adds significant new public API surface to TUnit.Assertions but does not include updated .verified.txt snapshot files.

New public types added (6 classes):

• CollectionHasAtLeastAssertion<TCollection, TItem>
• CollectionHasAtMostAssertion<TCollection, TItem>
• CollectionHasCountBetweenAssertion<TCollection, TItem>
• AsyncEnumerableHasAtLeastAssertion<TItem>
• AsyncEnumerableHasAtMostAssertion<TItem>
• AsyncEnumerableHasCountBetweenAssertion<TItem>

New public methods added (9 methods):

• HasAtLeast, HasAtMost, HasCountBetween on both CollectionAssertionBase and AsyncEnumerableAssertionBase
• CheckHasAtLeast, CheckHasAtMost, CheckHasCountBetween on CollectionChecks

Per CLAUDE.md:

"Changes to source generator output or public APIs require running snapshot tests. Commit .verified.txt files."

The TUnit.PublicAPI/ directory contains 4 .verified.txt files that track the full public API of TUnit.Assertions (e.g., Tests.Assertions_Library_Has_No_API_Changes.DotNet9_0.verified.txt). None of these were updated in this PR, so the Tests.Assertions_Library_Has_No_API_Changes snapshot tests will fail.

To fix: Run the snapshot tests, then promote the generated .received.txt files to .verified.txt and commit them:

# After running snapshot tests, from the repo root:
for f in TUnit.PublicAPI/*.received.txt; do mv "$f" "${f%.received.txt}.verified.txt"; done
git add TUnit.PublicAPI/*.verified.txt

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Overall Assessment

Aside from the missing snapshot updates, the implementation is well-structured and follows the established manual pattern correctly — which is the right approach for collection assertions. The [GenerateAssertion] attribute (preferred for simple scalar assertions) is not applicable here because collection assertions require two generic type parameters (TCollection and TItem), instance methods on CollectionAssertionBase for proper .And/.Or chaining, and internal adapter types (ICollectionAdapter<TItem>, EnumerableAdapter<TItem>) — none of which are supported by the generator. The logic is correct, null/exception handling is consistent with existing assertions, and the CallerArgumentExpression usage is properly wired up.